The present invention relates to a method for optically transmitting signaling and control information and to a receiving device for signaling and control information.
For transmitting a signal carrying useful information, one or a plurality of optical frequencies (wavelengths) is used. If a plurality of optical frequencies is used, they can be considered separate channels. This method is characterized as wavelength multiplex or wavelength division multiplexing (WDM), access to individual channels being possible for at least two network elements. This is characterized as multiple access.
In the operation of optical wavelength multiplex networks, signaling and control information must be transmitted. This signaling and control information functions, on the one hand, for controlling and monitoring network elements and, on the other hand, network conditions can be thus represented and can be changed. A reference (Chawki, M. J. Tholey, V. Le Gac I: Management Protocol of a Reconfigurable WDM Ring,Network Using SDH Overhead Bytes; OFC 96 Technical Digest) describes the signaling and control information bring transmitted within the signal of the useful information (in-channel signaling). In addition, a reference (Draft Recommendation ITU-T G.MCS Geneva Jun. 1996) describes the signaling and control information being transmitted on an additional control channel, either on the same or a different medium. Finally, a reference (S. Johansson: Transport Network Involving a Reconfigurable WDM Layer, Journal of Lightwave Technology 14 (1996) 6, pp. 1341-1349) describes, in which the signaling and control information being transmitted with the assistance of so-called pilot tones. In this context, the pilot tones are transmitted in an electrical frequency range that is not used by the useful information.
In-channel signaling is disadvantageous in that the signaling and control information must be separated from the useful information signal by using additional electrical de-multiplexers and electrical multiplexers, it being necessary first to convert the optical signal in its entire bandwidth into an electrical signal. For processing the signaling and control information, a transceiver (transmitter/receiver) is additionally necessary at each network element.
For the use of additional control channels for the signaling and control information, the separation/addition of the control channels from the channels of the useful information using appropriate means is necessary. In addition, the use of a transceiver is required for the processing of the signaling and control information. Due to the multiple access, it is necessary to use a protocol, which controls the use of the transceiver, i.e., it determines what time the transceiver is to transmit or receive and/or which subscriber can use the transceiver.
The use of pilot tones in optical wavelength multiplex networks having varying hierarchical orders is difficult, since in optical wavelength multiplex networks information can be transmitted at varying transmission rates. Taking into account the entire network structure, no uniformly free frequency ranges for the transmission of pilot tones are generated in this way.
U.S. Pat. No. 5,619,492 describes an optical, local-area network (LAN) in which control information is transmitted in a data packet together with useful information. The entire data packet composed of control information and useful information is CDMA-coded. In a receiving part, the entire data packet is again CDMA decoded.
European Published Patent Application No. 0 367 452 describes an optical network in which information is transmitted in the CDMA method and in the wavelength multiplex method.
A reference (L. Nguyen et al.: xe2x80x9cAll-Optical CDMA with Bipolar Codes,xe2x80x9d ELECTRONICS LETTERS, Vol. 31, No. 6, Mar. 16, 1995, pages 469-470) describes a CDMA method in optical networks, bipolar codes being used.
A reference (L. Giehman et al.: xe2x80x9cThe Application of Code Division Multiple Access for Transport Overhead Information in Transparent Optical Networks,xe2x80x9d OPTICAL FIBER COMMUNICATION CONFERENCE AND EXHIBITION, Vol. 2, Feb. 22-27, optical networks, the CDMA-coded signaling and control information being impressed upon the signal of the useful information.
It is an object of the present invention to provide a method for optically transmitting signaling and control information in optical wavelength multiplex networks, requiring no free optical or electrical frequency range and not interfering with the useful information signal to be transmitted.
The object is achieved in the present invention in that the Code Division Multiple Access (CDMA) signal of the signaling and control information is impressed using addition onto the signal of the useful information, the modulation range of the CDMA signal being less than the modulation range of the signal of the useful information. In this context, signaling and control information can be impressed on a multiplex signal existing on an optical fiber (LWL) or on individual channels. By using the code multiplex method, it is possible in an advantageous manner to transmit information selectively to individual network elements or to all network elements using the signaling and control information within an optical wavelength multiplex network. As a result of the fact that the modulation range of the CDMA signal is smaller than that of the useful information signal, influencing the signal of the useful information is substantially avoided. In addition, it is advantageous that the control signal is independent of synchronization with regard to the useful signal. This means that the CDMA signal can be transmitted at a different transmission rate than the useful information. For the detailed description of the CDMA method, two references are noted (R.C. Dixon: xe2x80x9cSpread Spectrum Systemsxe2x80x9d; John Wiley and Sons, 1996 and A. J. Viterbi: xe2x80x9cCDMA Principles of Spread Spectrum Communicationsxe2x80x9d; Addison-Wesley, 1995).
Furthermore, it is advantageous that an identification (addressing) of a network element can take place via a specifiable code. This code can be unipolar or bipolar, or, if appropriate, also multi-step. In the use of the CDMA method, one bit of the signaling and control information to be transmitted is linked to a code word so that a code sequence arises which is composed of a plurality of chips. The individual bits of the code sequence are termed xe2x80x9cchipsxe2x80x9d so as to avoid any confusion with the bits of the signaling and control information. By assigning the code words to individual network elements, the latter can be addressed. Thus, in an advantageous manner, it is possible to address individual network elements. For this purpose, a corresponding code word is to be clearly assigned using software to the network elements to be addressed. To be able to address a plurality of network elements of an optical wavelength multiplex network, a multiplicity of code words of this type may be employed in each case, however, at least one code word being clearly assigned to a network element. For example, it is nevertheless also possible to assign to code words to a network element representing, in this case, the information xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d. It is advantageous if the individual code words of the corresponding network elements are orthogonal to each other, a plurality of code words then constituting a so-called code family. By using a corresponding chip quantity (code word length), it is possible to be able to address virtually any number of network elements.
If a CDMA-coded bit of signaling and control information is transmitted to a network element that has a corresponding code word in its software, the bit can be identified in that, first, both code words are multiplied, with respect to chips, and then the individual multiplication results are added. Thus, for example, in a four-chip code word, the maximum sum possible is 4. As a result of transmission errors or noise, it is possible that individual chips are not clearly recognized or even are lost. The loss of a chip means that as the sum only the amount of 3 is calculated. However, here, too, the result is clear, since, for example, a multiplication with respect to chips and subsequent addition using a false code word that is orthogonal to the correct one, yields a result of 0. In this manner, a clear assignment of a bit of signaling and control information to a network element is possible. In addition, through the multiplication with respect to chips and subsequent addition, there results a so-called coding gain, since the original bit having the value of 1 receives the value 4 through the CDMA coding and the subsequent decoding. In this way, the signal-to-interference ratio is improved, i.e., a coding gain is achieved. In addition, it is possible in an advantageous manner by selecting a specifiable code to transmit in a particular manner particularly important signaling and control information.
In according to the present invention in impressing the signal of the signaling and control information on an envelope curve of the useful signal through intensity modulation, a small modulation index is selected.
In a further preferred embodiment of the method, provision is made that the CDMA-coded signal of the signaling and control information and the signal of the useful information are generated in a common network element as a summing signal. This means that both the CDMA-coded signal as well as the signal of the useful information are generated in a common transmitter (modulator). This happens as a result of the fact that the signals that are available at first as electrical pulses are converted into optical signals using an appropriate device, in the process the CDMA-coded signal being already impressed upon the signal of the useful information. Alternatively, provision can be made that the CDMA-coded signal of the signaling and control information is impressed on the signal of the useful information generated in a network element using a further network element.
The summing signal that arises in this manner including the CDMA-coded signal and the signal of the useful information, can be received by a further network element. A plurality of summing signals may be combined into at least one wavelength multiplex signal.
In according to the present invention provision is made that at the input of a network element, for generating the summing signal or wavelength multiplex signal, a supplemental signal identical to it is generated. This occurs preferably using a so-called monitor coupler. The monitor coupler can be assigned to an optical fiber, which carries the wavelength multiplex signal. An optical filter element can be arranged advantageously downstream of the monitor coupler, the filter element then filtering out of the wavelength multiplex signal at least one summing signal of an optical channel. Alternatively, provision can be made that the monitor coupler be assigned to an optically selective element, in which the wavelength multiplex signal is divided into individual summing signals in accordance with the wavelengths employed. The monitor coupler is then assigned to at least one optical fiber that carries a summing signal. In this manner, it is possible to dispense with the filter element just described. This means that the summing signal can be relayed directly from the monitor coupler to a CDMA decoder.
The currently existing summing signal is detected by the CDMA decoder. For this purpose, the decoder is configured such that it only detects the signals of the signaling and control information. This is possible due to the fact that the useful information is transmitted at a significantly higher transmission rate (M-bit range) than the signaling and control information (k-bit range). In the CDMA decoder, the transmitted code words are compared with the code word stored in the CDMA decoder. In the event of agreementxe2x80x94 as described abovexe2x80x94 the original bit sequence of the signaling and control information can be recovered, the latter being converted into the electrical area. In this manner, a processing is possible in a suitable downstream device. In this processing, on the one hand, control information for a network element is evaluated, whereas, on the other hand, it is possible that further signaling and control information can be generated so that further network elements can be controlled or monitored. Subsequently, this new signal of the signaling and control information may be again CDMA coded and may be impressed on the signal of the useful information. In an advantageous manner, the conversion of the useful information into the electrical range can be dispensed with, which leads to a simplification and cost reduction of the entire network.
In addition, the present invention relates to a device for receiving signaling and control information, which is characterized in that the receiving device of the signaling and control information has at least one monitor coupler and one signaling device. The monitor coupler, according to the present invention, generates a supplemental signal that is identical to the received signal that includes signaling and control information and useful information, the supplemental signal being fed to the CDMA decoder, which filters the signaling and control information from this signal and, subsequently, decodes it, and in that the decoded signaling and control information is fed to the signaling device, which controls the network element. In particular, as a result of the fact that the monitor coupler generates this supplemental signal from which in the CDMA decoder the control information is filtered out, it is possible to dispense with a recovery of the useful information, since the signaling and control information are impressed upon the useful signal and therefore can easily be separated from it.
In another embodiment the device according to the present invention, provision is made that the device have assigned to it a transmitting device including a code multiplex coder. In this way, it is possible to code once again the information generated in the signaling device so that it can subsequently be impressed on the signal of the useful information through addition using a modulator.